FIG. 1 depicts a prior-art projection apparatus comprising a light source 1, an illumination lens 2, a stop 3 defining an incident-side aperture 3a and an exit-side aperture 3b, a front lens group 4, a reflective spatial light modulator (RSLM) 5 (such as a "light valve" as known in the art), a rear lens group 6, and a surface ("screen") 7 on which reflected modulated light from the RSLM 5 creates a viewable image. The front lens group 4 and the rear lens group 6 are arranged on an optical axis z and constitute a "projection optical system" of the apparatus. The light source, illumination lens, and incident-side aperture comprise an "illumination optical system" of the apparatus. A parallel illumination light flux 1a, produced by the light source 1, is focused by the illumination lens 2 so as to converge at the incident-side aperture 3a. The front lens group 4 refracts the illumination light flux 4a diverging from the incident-side aperture 3a to produce a substantially parallel incident light flux (rays 4b) that impinge, at an angle to the optical axis z, on the RSLM 5. The RSLM 5 produces, from the incident light flux, a reflected modulated light flux (rays 8) that is refracted by the front lens group 4 to converge at the exit-side aperture 3b. The rear lens group 6 refracts, and thus projects, the modulated light flux (rays 6a) diverging from the exit-side aperture 3b to the screen 7 or analogous viewing surface that forms a viewable image from the modulated light flux (rays 6b).
A schematic cross section of a representative RSLM 5 according to the prior art, shown in FIG. 2, comprises a light-modulation layer 5b situated between a reflective surface 5a and a plane-parallel transparent layer 5c. The reflective surface 5a is substantially planar in profile and is parallel to the transparent layer 5c. The transparent layer 5c has a substantially planar surface 5d.
FIG. 3 depicts details of the stop 3 utilized in the apparatus of FIG. 1. The stop 3 comprises a light-shielding body that defines an incident-side aperture 3a, through which the illumination light flux passes to the RSLM 5, and an exit-side aperture 3b through which the modulated light flux passes to the screen 7. Both apertures 3a, 3b are arranged symmetrically around the optical axis z (extending normal to the plane of the page) of the lens groups 4, 6.
Referring further to FIG. 2, the light-modulation layer 5b of the RSLM 5 is disposed closer than the reflective surface 5a to the projection optical system 4, 6. Also, the transparent layer 5c, which can be protective glass or the like, is situated closer to the projection optical system 4, 6 than the light-modulation layer 5b.
FIG. 1 depicts two categories of light reflected from the RSLM 5. "Signal" light (rays 8, 6a, 6b denoted by solid lines) represents light that, after having passed as incident light through the transparent layer 5c and the light-modulation layer 5b, reflects from the reflective surface 5a and passes again through the light-modulation layer 5b and the transparent layer 5a. "Ghost" light (indicated by dashed lines, e.g., rays 9) represents light that, as incident light, is reflected from the surface 5d of the transparent layer 5c without penetrating to the light-modulation layer 5b or the reflective surface 5a. Since the reflective surface 5a and the transparent layer 5c are planar and parallel to each other, the signal light 8 and the ghost light 9 are parallel to each other between the RSLM 5 and the front lens group 4. The front lens group 4 causes the signal light 8 and the ghost light 9 to converge at the same point at the exit-side aperture 3b. Unfortunately, however, because the signal light and ghost light both pass through the exit-side aperture 3b, both propagate to the screen 7, where the ghost light diminishes image contrast.
A conventional RSLM 5 that utilizes scattering, such as an RSLM employing a polymer dispersion-type liquid crystal (PDLC) element as the light-modulation layer, requires that incident light be at an angle of incidence significantly greater than zero degrees to adequately separate "exit" light (i.e., light propagating from the RSLM) from incident light (i.e., light propagating to the RSLM). Unfortunately, this can complicate the construction of a projection apparatus employing the RSLM.